1. Field of the Invention
This invention relates to processes and apparatus for the fluidized contacting of catalyst with hydrocarbons. More specifically, this invention relates to processes and apparatus for stripping entrained or adsorbed hydrocarbons from catalyst particles.
2. Description of the Prior Art
A variety of processes contact finely divided particulate material with a hydrocarbon containing feed under conditions wherein a fluid maintains the particles in a fluidized condition to effect transport of the solid particles to different stages of the process. Catalyst cracking is a prime example of such a process that contacts hydrocarbons in a reaction zone with a catalyst composed of finely divided particulate material. The hydrocarbon feed fluidizes the catalyst and typically transports it in a riser as the catalyst promotes the cracking reaction. As the cracking reaction proceeds, substantial amounts of hydrocarbon, called coke, are deposited on the catalyst. A high temperature regeneration within a regeneration zone burns coke from the catalyst by contact with an oxygen-containing stream that again serves as a fluidization medium. Coke-containing catalyst, referred to herein as spent catalyst, is continually removed from the reaction zone and replaced by essentially coke-free catalyst from the regeneration zone. Fluidization of the catalyst particles by various gaseous streams allows the transport of catalyst between the reaction zone and regeneration zone. Methods for cracking hydrocarbons in a fluidized stream of catalyst, transporting catalyst between reaction and regeneration zones and combusting coke in the regenerator are well known to those skilled in the art of FCC processes. To this end, the art is replete with vessel configurations for contacting catalyst particles with feed and regeneration gas, respectively.
A majority of the hydrocarbon vapors that contact the catalyst in the reaction zone are separated from the solid particles by ballistic and/or centrifugal separation methods within the reaction zone. However, the catalyst particles employed in an FCC process have a large surface area, which is due to a great multitude of pores located in the particles. As a result, the catalytic materials retain hydrocarbons within their pores, upon the external surface of the catalyst and in the spaces between individual catalyst particles as they enter the stripping zone. Although the quantity of hydrocarbons retained on each individual catalyst particle is very small, the large amount of catalyst and the high catalyst circulation rate which is typically used in a modern FCC process results in a significant quantity of hydrocarbons being withdrawn from the reaction zone with the catalyst.
Therefore, it is common practice to remove, or strip, hydrocarbons from spent catalyst prior to passing it into the regeneration zone. Improved stripping brings economic benefits to the FCC process by reducing xe2x80x9cdelta cokexe2x80x9d. Delta coke is the weight percent coke on spent catalyst less the weight percent coke on regenerated catalyst. Reducing delta coke in the FCC process permits a lowering of the regenerator temperature. Consequently, more of the resulting, relatively cooler regenerated catalyst is required to supply the fixed heat load in the reaction zone. The reaction zone may hence operate at a higher catalyst-to-feed or catalyst-to-oil (C/O) ratio. The higher C/O ratio increases conversion which increases the production of valuable products. Accordingly, improved stripping results in improved conversion. A stripping operation that reduces the production of delta coke by 0.05 wt-% can lower the regenerator temperature by 8xc2x0 to 11xc2x0 C. (15xc2x0 to 20xc2x0 F.) and permit a C/O ratio increase in the range of 6%. The corresponding improvement in conversion yields 0.6 to 0.7 vol-% more gasoline as well also increasing the yield of desired light products. Additionally, stripping hydrocarbons from the catalyst also allows recovery of the hydrocarbons as products.
The most common method of stripping the catalyst passes a stripping gas, usually steam, through a flowing stream of catalyst, counter-current to its direction of flow. Such steam stripping operations, with varying degrees of efficiency, remove the hydrocarbon vapors which are entrained with the catalyst and adsorbed on the catalyst. Contact of the catalyst with a stripping medium may be accomplished in a simple open vessel as demonstrated by U.S. Pat. No. 4,481,103 B1.
The efficiency of catalyst stripping is increased by using vertically spaced baffles to cascade the catalyst from side to side as it moves down a stripping apparatus and counter-currently contacts a stripping medium. Moving the catalyst horizontally increases contact between the catalyst and the stripping medium so that more hydrocarbons are removed from the catalyst. In these arrangements, the catalyst is given a labyrinthine path through a series of baffles located at different levels. Catalyst and gas contact is increased by this arrangement that leaves no open vertical path of significant cross-section through the stripping apparatus. Further examples of these stripping devices for FCC units are shown in U.S. Pat. No. 2,440,620 B1, U.S. Pat. No. 2,612,438 B1, U.S. Pat. No. 3,894,932 B1, U.S. Pat. No. 4,414,100 B1 and U.S. Pat. No. 4,364,905 B1. These references show the typical stripping vessel arrangement having a stripping vessel, a series of outer baffles in the form of frusto-conical sections that direct the catalyst inwardly onto a series of inner baffles. The inner baffles are centrally located conical or frusto-conical sections that divert the catalyst outwardly onto the outer baffles. The stripping medium enters from below the lower baffles and continues rising upwardly from the bottom of one baffle to the bottom of the next succeeding baffle. Variations in the baffles include the addition of skirts about the trailing edge of the baffle as depicted in U.S. Pat. No. 2,994,659 B1 and the use of multiple linear baffle sections at different baffle levels as demonstrated in FIG. 3 of U.S. Pat. No. 4,500,423 B1. A variation in introducing the stripping medium is shown in U.S. Pat. No. 2,541,801 B1 where a quantity of fluidizing gas is admitted at a number of discrete locations. Baffles can also include an upstanding weir on the edge of the baffle adjacent the downcorner.
Currently in stripping vessels for FCC units, the baffles are typically oriented to have an angle of 45xc2x0 with respect to the horizontal. The sloped baffles assure that catalyst moves off the tray down to the next level in the stripping vessel. Sloped baffles generate a differential pressure head between holes that are lower in elevation on a baffle compared to the holes which are higher in elevation on the baffle. Because the pressure is going to be greater at lower elevations on the baffle, the velocity through the jets on the baffle will be greater at higher elevations on the baffle. Sloped baffles typically have several rows of holes near the bottom portion of the baffle. These trays perform well at low catalyst fluxes but have had very large stagnant zones occupying nearly two-thirds of the stripper vessel.
Residence time of catalyst in the stripper is expected to be a significant factor because the stripper is a secondary reactor where adsorbed heavy hydrocarbons undesirably continue to react producing coke and light hydrocarbons such as hydrogen, methane, ethane, ethylene, propane and propylene. The greater the amount of stagnant or dead zones, the greater the amount of heavy hydrocarbons that will continue to react.
U.S. Pat. No. 5,531,884 B1 attempts to utilize a tubular downcorner near the central region of the baffle to improve flux capabilities. However, such tubular downcorners do not serve well to mix catalysts and stripping fluid but rather passes either one without mixing with the other at particular times resulting in a degradation of stripping efficiency.
U.S. application Ser. No. 09/746,751 filed Dec. 21, 2000 improves the baffle configuration by spreading the holes out on the baffles such that equal gas is supplied to equal areas of the tray. This improvement all but eliminated the stagnant zones. The improved baffle exhibited very high flux capabilities with improved stripping performance at higher fluxes. However, at low fluxes, the improved baffle configuration performed with less or comparable efficiency than the conventional baffle with rows of holes near the bottom edge of the baffle in cold flow model evaluations. The provision of equal gas to equal areas of the baffle does not work in the case of low fluxes because there is insufficient catalyst momentum to sweep the bulk of the gas emitted from the openings in the baffle across the downcorner space to beneath the superjacent baffle.
Accordingly, it is an object of this invention to increase the efficiency of stripping in a baffle style stripper at low catalyst flux rates.
It is an additional objective of any new stripper design to minimize the amount of stagnant zones at low fluxes while maintaining the benefits of good catalyst stripping throughout the FCC process unit.
It is a further objective of this invention to decrease delta coke by more efficient catalyst stripping.
It has now been found that providing a baffle-style stripper with openings configured to promote a greater volumetric flow rate of stripping medium to move through the bottom section of the baffle than through the top section of the baffle minimizes the generation of dead zones at low fluxes. Concentrating stripping medium flow nearer to the downcorner helps smaller fluxes of catalyst to sweep stripping media across the downcorner space to beneath the superjacent baffle. The horizontal movement of catalyst and stripping medium fosters efficient stripping.
For applications where the catalyst flux through the stripping zone is relatively low, the hole distribution arrangement of this invention provides substantial benefits. For lower flux applications, which typically refer to a flux below 90,000 lbs/hr/ft2 (439,380 kg/hr/m2) of stripper area, the catalyst flow across the stripping baffles tends to be greatest towards the lower portion of the baffles. Accordingly, a biasing of the stripping gas flow towards the lower portion of the baffle can particularly benefit low catalyst flux applications. In such cases, increasing the open hole area towards the lower portion of the baffle beyond that which would provide a uniform volumetric gas delivery across the baffle assures that a greater volumetric delivery of gas occurs over the lower portion of the sloped baffle.
Accordingly, in one embodiment, the present invention relates to a process for the stripping of entrained and/or adsorbed hydrocarbons from particulate material. The process comprises contacting particles with a hydrocarbon stream. Hydrocarbons are disengaged from the particles after contact with the hydrocarbon stream to produce a stream of contacted particles containing hydrocarbons. The contacted particles are then passed downwardly over a plurality of sloped stripping baffles, each baffle having a top section proximate a top edge of the baffle and a bottom section proximate a bottom edge of the baffle. The top section and the bottom section of the baffle are demarcated by an imaginary line extending laterally on the baffle and substantially parallel to one of the top edge, the bottom edge and an imaginary line bifurcating the baffle into equal areas. The top section and the bottom section of the baffle each also include a plurality of openings. A stripping fluid is discharged upwardly through the openings of the baffles such that a volumetric flow rate of stripping fluid moving through the bottom section of the baffle is greater than a volumetric flow rate of stripping fluid moving through the top section of the baffle. The stripping fluid strips hydrocarbons from the particulate material. Stripping fluid, stripped hydrocarbons and stripped particles are then recovered from the stripping baffles.
In another embodiment, the present invention relates to an apparatus for the stripping of entrained and/or adsorbed hydrocarbons from particulate material. The apparatus comprises a stripping vessel and at least one port defined by the stripping vessel for receiving particles that contain entrained or adsorbed hydrocarbons from the contact of the particles with a hydrocarbon stream and for withdrawing stripping fluid and stripped hydrocarbons from the stripping vessel. A plurality of sloped stripping baffles is spaced apart vertically over at least a portion of the stripping vessel height with each baffle having a sloped surface. Each baffle has a top section proximate a top edge of the baffle and a bottom section proximate a bottom edge of the baffle. The top section and the bottom section are demarcated by an imaginary line extending laterally on the baffle and substantially parallel to one of the top edge, the bottom edge and an imaginary line bifurcating the baffle into equal areas. A plurality of openings is on the top section of the baffle, and a plurality of openings is on the bottom section of the baffle. A ratio of the total area of openings to the area of the section of the baffle is greater in the bottom section of the baffle than in the top section of the baffle. The apparatus also comprises at least one fluid inlet for passing a stripping fluid to the underside of at least one stripping baffle for stripping hydrocarbons from the particulate material, and at least one particle outlet for recovering stripped particles from the stripping baffles.
In a further embodiment, the present invention relates to a process for the stripping of entrained and/or adsorbed hydrocarbons from particulate material, wherein the entrained and/or adsorbed hydrocarbons are from the fluidized catalytic cracking (FCC) of an FCC feed with a particulate material comprising an FCC catalyst. The process comprises contacting an FCC feed with FCC catalyst to provide a mixture of FCC catalyst and FCC feed and to convert the FCC feed while depositing coke on the FCC catalyst. Converted FCC feed is disengaged from the FCC catalyst to produce a stream of disengaged catalyst particles containing hydrocarbons. The disengaged catalyst particle stream is passed into a stripping zone, and the stream of catalyst particles is passed downwardly over a plurality of vertically sloped stripping baffles in the stripping zone. Each baffle has a top section proximate a top edge of the baffle and a bottom section proximate a bottom edge of the baffle. The top section and the bottom section are demarcated by an imaginary line extending laterally on the baffle and substantially parallel to one of the top edge, the bottom edge and an imaginary line bifurcating the baffle into equal areas. A stripping fluid is discharged upwardly through a plurality of openings in the top section of the baffle and a plurality of openings in the bottom section of the baffle. The openings are distributed to provide a greater volumetric flow rate of stripping fluid to the lower portion of the sloped surface than to the upper portion of the sloped surface. Stripping fluid and stripped hydrocarbons that pass upwardly from the stripping baffles are recovered. Stripped FCC catalyst that passes downwardly from the stripping baffles is recovered. Stripped FCC catalyst is passed to a regeneration zone to remove coke from the FCC catalyst. FCC catalyst is then returned from the regeneration zone for contact with the FCC feed.
Additional objects, embodiments, and details of this invention are given in the following detailed description of the invention.